The solar vacuum tube with heat pipe is the most efficient solar water heater on the market. Heat pipe technology and vacuum tubes are nothing new. In fact, the International Space Station uses both technologies at the same time. Most laptop computers use heat pipes to cool the motherboard, while vacuum tubes have been used in electronic products such as TVs and radios. In fact, vacuum tubes are still the choice of high-end amplifiers. So, how do these two technologies work in solar water heating systems? Let's start with solar vacuum tubes or also called evacuated tubes.
What Is Vacuum Tube?
The vacuum tube is made of borosilicate glass, which has a very low iron content, so it can pass more solar radiation than ordinary glass, so it is also very strong. The manufacturing method of the glass tube is the same as the manufacturing method of the thermos, which has an outer glass layer and an inner glass layer. In between, the air has been removed or "evacuated" to create a vacuum. Since there is no air inside, heat cannot be transferred to either side. The glass inner tube is coated with a patented 3-layer copper, aluminum and aluminum/nitrogen layer process. These selective coatings are responsible for absorbing and transmitting the solar thermal radiation, thereby affecting the performance of the tube. Since there is a vacuum between the two, the heat cannot be dissipated, making the efficiency of these tubes 97%. During the manufacturing of these tubes, a barium absorbent is installed to absorb any gas to ensure that the vacuum is maintained. The getter is also used to indicate the integrity of the impaired vacuum. If the vacuum of the test tube breaks, the barium getter will cause the metal coating on the bottom of the test tube to turn white, indicating that it needs to be replaced.
After the selective coating absorbs the heat energy of the sun, the energy is transferred to the central heat pipe through the aluminum fins, which are wrapped inside the vacuum tube and completely contact the selective coating. The center of the aluminum heat sink is a copper heat pipe. The heat pipe is a long tube that runs through the entire length of the vacuum tube. There is a small amount of liquid inside the hollow of the tube. The inside of the copper heat pipe is also in a vacuum state, but the vacuum is different from the vacuum in the evacuated pipe. Due to this vacuum, the liquid boils at a temperature of about 200°C. 30 degrees Celsius (86 degrees Fahrenheit). This is the main principle behind the heat pipe technology. Compared with sea level, water under vacuum boils at a lower temperature, while water under high pressure boils at a higher temperature.
When the heat pipe is preheated, the liquid is rapidly heated inside the heat pipe and becomes steam. The steam rises along the tube until it reaches the condenser bulb, where its heat is transferred to the solar heating fluid through the heat exchanger in the manifold. In the condenser bulb, the steam loses energy and returns to liquid. Gravity causes the liquid to flow downwards back to the inner wall of the inner heat pipe, and the cycle repeats.
The copper heat pipe in each tube is inserted into the copper sleeve in the sealed manifold. This heat is indirectly transferred to the solar heating liquid through the conductivity. Therefore, in this way, the manifold is independent of the heat pipe, and no leakage occurs even if the heat pipe is removed. The manifold is highly insulated, and there is a small sleeve for inserting the temperature sensor in the copper manifold. In this way, the internal temperature of the heating fluid is transferred back to the digital controller. If the temperature in the collector is higher than the water in the water tank or pool, the controller sends a signal to notify the pump to turn on and circulate the heating fluid. In the evening when the sun goes down, the manifold will cool to a temperature lower than that of the water or pool, which will instruct the pump to stop.
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